A rare but potentially life-threatening tropical fungus is spreading through the Pacific Northwest, researchers have reported.

The culprit is a new strain of the Cryptococcus gatti fungus, and is known to have been lethal in 25 percent of the reported human infections. C. gatti usually only infects transplant and AIDS patients and people with otherwise compromised immune systems, but the new strain is genetically different, the researchers said. “This novel fungus is worrisome because it appears to be a threat to otherwise healthy people” [Reuters], says lead researcher Edmond Byrnes.

However, scientists aren’t sounding a public health alert because the death toll is still very small–in the United States, five of the 21 people who contracted the fungus in the have died.

We wouldn’t recommend that people change their habits in any way,” Julie Harris, PhD, MPH, a staff epidemiologist with the CDC, tells WebMD. “We wouldn’t recommend people stay indoors or don’t go hiking or don’t go outdoors.”

The fungus species triggering the infection is Cryptococcus gattii, which can cause pneumonia or meningitis. But the infection ”simply is not common enough for people to warrant changing behavior,” Harris says. “It’s still very rare. People should be concerned but not alarmed.”

At a news briefing Friday, Katrina Hedberg, MD, MPH, interim state epidemiologist for the Oregon Department of Health Services Public Health Division, told reporters that it’s also rare that people exposed to the fungus end up getting sick.

While the CDC wouldn’t specify the number of deaths, citing incomplete data, Hedberg says that ”of the 50-plus cases, around 10 of them have died.”

Twelve of those 50 cases, including three deaths, have been in the state of Washington, according to Nicola Marsden-Haug, MPH, an epidemiologist with the Washington State Department of Health, Shoreline.

Marcia Goldoft, MD, a medical epidemiologist with the department, urges people to keep the threat in perspective. “The benefits of outdoor activity and exercise far outweigh the risks of a rare disease such as C. gattii.”

To be fair, it was a scientist — Edmond Byrnes, a graduate student in microbiology and molecular genetics at Duke University — who triggered the fungus frenzy. His paper, published Thursday in PLoS Pathogens, reported that detailed genetic analyses had revealed that two strains of C. gattii, a fungus that typically lives in trees and soil, had become hypervirulent. Byrnes’ paper looked at six deaths and 15 other infections in people, and 21 cases in animals that had occurred in the U.S. between 2005 and 2009.

But here’s what you really need to know: “These infections are still rare, and from an overall health perspective, I don’t think anyone should be concerned, but should just be aware that it is increasing geographically and incidence-wise in [the Pacific Northwest],” says Byrnes. “For the average person, I don’t think this is anything to be too worried about.”

C. gatti is normally found in tropical climates in South America, Australia and Papua New Guinea. In these endemic regions, it tends to favor eucalyptus trees and, according to Julie Harris, an epidemiologist in the mycotic diseases branch of the Centers for Disease Control and Prevention, rates of infection among people are relatively low.

The fungus was somehow carried from the southern hemisphere to North America, where it was found on Vancouver Island in 1999. (It was rare — at its peak, between 2002 and 2005, there were 36 cases per million population per year in the region reported to health officials.) One of the new strains of highly virulent C. gattii was determined to have originated on Vancouver Island; the other is thought to have emerged in Oregon, possibly from a strain that had spread south from British Columbia. In lab animals, Byrnes reports, these two strains are 100% lethal, causing death within three weeks. That’s reason for concern from a scientific standpoint, he says, since other known strains of the fungus are not as deadly. But, again, the fungus is so rare in the real-world, that from a public-health perspective, there’s no need for alarm.

One of the lingering questions the researchers were left with was whether the fungus is becoming more virulent as it spreads. The answer hinges in part on how the fungi reproduce (since fungi can do it in a number of ways). It looks as if the new strains of C. gattii are getting it on with opposite sex and same sex partners. These matings appear to result in spores that are even more deadly to living creatures.

If C. gattii keeps having sex and spreading, its next victims will mostly likely be in Northern California, where the weather is very similar to Oregon. It’s unlikely to expand eastward, due to the freezing winters.

The mutation “is causing major illness in the region, and it’s different from what’s causing disease on Vancouver Island,” says Christina Hull, PhD, an assistant professor of medical microbiology and immunology at the University of Wisconsin School of Medicine and Public Health, in Madison. “It supports the idea that this is a recent change in the organism,” she adds. “That’s a little more unnerving than what people had thought before.”

Scientists don’t know what risk factors there are for the disease. The good news, though, is that it’s unlikely to travel outside the region, and even then its virulence will likely decline over time.